Recent years have seen the expenditure of considerable effort to reduce the size of various heat exchangers, particularly those used in air conditioning systems and even more particularly, those used in vehicular air conditioning systems. At the same time, heat exchange efficiency cannot be lost as the heat exchangers are down-sized.
Size reduction typically results in weight reduction which, in a given vehicle, can improve fuel economy. Furthermore, a size reduction permits greater freedom in designing the envelope, i.e. vehicle body, in which the heat exchanger will be housed. This, in turn, allows the achievement of designs that are more aerodynamically clean. This, in turn, provides another source of fuel savings.
Moreover, reduction in size frequently means that the refrigerant charge may be reduced as well. Given that most common refrigerants today are chloro fluoro carbons or so called "CFC's", and given further that CFC's have a deleterious effect on the ozone layer, a reduction in refrigerant charge reduces the amount of refrigerant that is potentially available to leak to the atmosphere.
Hydrochlorofluoro carbons (HCFC's) suggested a replacement for CFC's have adverse global warning potential.
In the quest for size and weight reduction, some effort has been focused on header construction. Headers are, of course, necessary to properly distribute the heat exchange fluid to one or more flow paths in which heat is exchanged. However, the headers themselves do not contribute significantly to heat exchange between fluids. Thus, to the extent that a header occupies part of a given envelope, it represents a reduction in that part of the envelope that may be devoted to the components primarily responsible for heat exchange, namely, tubes interconnecting the headers and fins extending between the tubes or runs of a tube.
At the same time, headers may be required to withstand substantial pressures. For example, it is not uncommon to design headers for use with condensers in vehicular air conditioning systems to withstand pressures approaching 2000 psi, even though such a pressure is substantially in excess of normal working pressure within the system.
To meet these and other objectives, the use of tubular headers has been proposed for heat exchangers used in vehicular air conditioning systems. See, for example, U.S. Pat. No. 4,998,580, issued Mar. 12, 1992, to Guntly et al., the details of which are herein incorporated by reference. This heat exchanger utilizes cylindrical tubes as headers. The tubes are slotted at regular intervals and then flattened tubes are brazed into the slots with fins extending between adjacent flattened tubes.
This construction results in a light weight, low volume, heat exchanger capable of withstanding substantial pressures and having a high efficiency. It appears to be well on its way to recognition as the state-of-the-art automotive air conditioning condenser.
At the same time, further efforts are being made to reduce header size so as to maximize the amount of tube and fin surface that may be contained in a given envelope. A number of recent patents, including U.S. Pat. No. 4,903,389 issued Feb. 27, 1990 to Wolf, have disclosed what might be termed "laminated header constructions". In these constructions, typically a minimum of three relatively thin plates are sandwiched together. One plate on the exterior of the sandwich may be a cap plate while the opposite exterior plate in the sandwich may be a tube plate, receiving the ends of flattened tubes or the like. The center plate may contain a series of channels interconnecting the various slots in the tube plate to define, with the tube plate and the cap plate, a header chamber.
This type of construction is highly pressure resistant and is of minimal volume, allowing an increase in the percentage of any given envelope that may be devoted to tubes and fins. However, the use of three or more plates is not as efficient from the weight standpoint as other types of headers.
As a consequence, two lamination headers have evolved. In a typically two lamination header, one plate serves as a tube plate just as in a conventional laminated header. The other plate is stamped to include one or more concave channels or troughs which face the tube plate. This plate may be termed a tank plate and the same is bonded to the tube plate and sealed thereto. As a consequence, the channels in the tank plate serve as header chambers' interconnecting slots in the tube plate. In the usual case, a two lamination header will occupy no more space than a conventional laminated header and yet will weigh considerably less since it permits the elimination of at least one plate in the conventional laminated header construction.
Unfortunately, two lamination headers cannot be used efficiently in heat exchangers employed as evaporators. Typically, some part of the sides of the channel stamped in the tank plate are caused to overly the slots in the tube plates so that tubes received in these slots have their excursion into the header chamber limited by interference with sides of the channel or channels formed in the tank plate. In the usual case, the ends of the tubes will extend a small distance into the header chamber and thus each tube end acts as a small fence around the end of the associated flow path defined by tube itself. Where attempts are made to eliminate this small fence, part of the tank plate that fits flush against the tube plate must overly each slot and thus tends to partially occlude the tube receiving opening.
In either case, flow of refrigerant within the header chamber to the tubes is interfered with and one consequence is that the distribution of the refrigerant, which will be at least partially in the liquid phase when the heat exchanger is being used as an evaporator, is poor from one side of the evaporator to the other. And, the poor distribution, in turn, results in inefficient heat transfer.
This difficulty cannot be easily solved by return to the use of tubular headers as disclosed in the previously identified U.S. Pat. No. 4,998,580. Those skilled in the art will readily recognize that the tube ends will also act as fences in such a construction, simply because the side of the tubular headers through which they enter is not flat.
The present invention is directed to solving one or more of the above problems.